How platelets support homeostasis in the vascular system is a paradigm-central question in hemostasis and thrombosis. The importance of the platelet glycoprotein (GP) Ib-IX/von Willebrand factor axis in this process is well-established and represents an initiating event for platelet adhesion to a damaged vascular surface. Our long-term goals are to establish the molecular mechanism(s) by which platelet receptor function, specifically GP Ib-IX (adhesion) and the collagen receptor, GP VI (activation), maintain vascular homeostasis in normal physiology and contribute to disease states. Mice with targeted gene deletions and expressed variant surface receptors have been generated to examine the unique biological properties of the megakaryocyte and platelet in vivo. An example of the utility of these models resides in the fact that for several decades an interaction between platelet GP Ib-IX and thrombin has been described with little insight as to the physiologic relevance, if any, of the binding. We present new data characterizing a mouse model with a Tyr276 to Phe276 substitution within a key residue of the GP Ib/thrombin contact site. Preliminary studies establish the in vivo importance of this residue in forming a platelet-rich thrombus. We propose to characterize the in vivo relevance of thrombin binding to platelet GP Ib-IX using models of thrombus formation testing the hypothesis that GP Ib-IX/thrombin interactions stabilize a growing thrombus (Aim 1). We present an animal model of platelet-type von Willebrand disease (Pt-vWD) to test the hypothesis that Pt-vWD mimics GP Ib-IX blockade and presents neoepitopes, analogous to the ligand-induced binding sites characterized for platelet integrin receptors (Aim 2). The congenital absence of platelet GP Ib-IX results in the Bernard-Soulier syndrome (BSS) and presents with a bleeding phenotype and circulating giant platelets. We propose a proteomics approach to define the unique megakaryocyte and platelet phenotypes generated by a BSS (Aim 3). These studies test the hypothesis that the BSS megakaryocyte and platelet contain altered protein profiles leading to a secondary pleiotropy that is the BSS and may help elucidate the unknown molecular basis for other giant platelet phenotypes. Finally we propose to determine the relevance of GP Ib-IX (platelet adhesion) and GP VI (platelet activation) in development of atherosclerotic lesions and thrombosis using a model of LDL-receptor deficiency (Aim 4). We propose to test the hypothesis that platelet adhesion and activation mediated by GP Ib-IX and/or GP VI are important contributors to inflammation. Completion of each aim further defines the biological relevance of GP Ib-IX and GP VI in thrombosis and megakaryocytopoiesis but also considers platelet relevance beyond the platelet paradigm in hemostasis and thrombosis. PROJECT NARRATIVE The role of circulating blood platelets is to prevent blood loss. However, in disease platelets can cause the pathologic disruption of blood that leads to organ damage and failure. The most common event is a myocardial infarction in the heart and this is the leading cause of death in the U.S. Our studies use mouse models to mimic those events leading to fatal thrombosis and characterize the molecular events associated with heart attack and stroke. An understanding of how platelets function in the process will lead to better therapeutic strategies and improved health in the U.S.